[RTC] remove unused asm/rtc.h includes from ARM RTC drivers

On ARM, asm/rtc.h only contains definitions for the predecessor to
the RTC class support.  RTC class drivers should not be including
this include.

Build tested on at91sam9rl and s3c2410 configurations.

Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>